sexpr: improve parser error messages
sexpr: improve parser error messages

Instead of repeating "Error" (CE_ERROR already prints one for us), let's
identify the subsystem generating the error message.

Signed-off-by: Josef 'Jeff' Sipek <a620f141433c70aa9e43778305a6c68cbfadfb25@josefsipek.net>

jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc